Nanostructured Magnesium Composite Coatings for Corrosion

Article pubs.acs.org/IECR

Nanostructured Magnesium Composite Coatings for Corrosion Protection of Low-Alloy Steels Robert V. Dennis,† Lasantha T. Viyannalage,†,§ Jeffrey P. Aldinger,† Tapan K. Rout,‡ and Sarbajit Banerjee*,† †

Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States Research & Development, Tata Steel Ltd., Jamshedpur, India 831007

Ind. Eng. Chem. Res. 2014.53:18873-18883. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 09/28/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: Corrosion of base metals represents a tremendous problem that has spurred a global search for cost-effective and environmentally friendly alternatives to current corrosion-inhibiting technologies. In this work, we report a novel sustainable hybrid Mg/poly(ether imide) (PEI) nanocomposite coating that provides corrosion protection to low-alloy steels at relatively low coating thicknesses and with reduced weight as compared to conventional metallic coatings. The coatings are constituted using Mg nanoplatelets dispersed within a polyamic acid matrix that is subsequently imidized on the steel substrate to form PEI. The coatings function through a combination of sacrificial cathodic protection (afforded by the preferential oxidation of the Mg nanoplatelets), anodic passivation through precipitation of corrosion products, and the inhibitive action of the PEI polymeric matrix. The use of nanostructured Mg allows for reduced coating thicknesses and a smoother surface finish, whereas the PEI matrix provides excellent adhesion to the metal surface. Based on potentiodynamic testing and prolonged exposure to saline environments, the novel coating materials significantly outperform galvanized Zn and Zn-rich primer coatings of comparable thickness.

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sub-15-μm film thicknesses of the active primer layer based on a combination of different modes of action and represents an attractive chrome- and zinc-free alternative for prolonged corrosion protection of low-alloy steels. For many years, the standard for sacrificial electroactive corrosion coatings has been Zn (applied either through electrogalvanization or via the more economical hot-dip process) due to its excellent barrier-type properties, which provides sheltering of the substrate from corrodant species, as well as sacrificial protection stemming from the differential in reduction potentials of the zinc and the steel substrate.7 The zinc coating of a steel sheet preferentially oxidizes prior to the iron of the steel substrate, thereby extending the usable lifetime of the steel component to the interval of time when the galvanic layer is depleted. Simonkolleite, Zn5(OH)8Cl2·H2O, which is the primary corrosion product for galvanized steel, along with ZnO further forms a passivating layer stable across a significant pH range. However, the reliance on cathodic protection and the concept that the extent of protection is entirely proportional to the thickness of the coating motivates the need to develop new coating materials that provide comparable (or enhanced) corrosion inhibition at much reduced coating thicknesses. Indeed, there are significant cost and weight penalties associated with the use of thick layers of Zn that have lent new urgency to the search for alternative coating

INTRODUCTION Combatting the corrosion of structural metal components remains one of the most daunting challenges of science and technology. The development of novel concepts for corrosion inhibition has acquired new urgency due to the renewed industrial emphasis on sustainability and green chemistry, and the coming of age of novel lightweight structural alloys.1,2 Corrosion represents a colossal drain on resources with some accounts estimating that prevention of corrosion and the repair/replacement of corroded components costs the U.S. economy as much as $276 billion per year.3 Coatings designed to inhibit corrosion typically function via one (or more) of the following modes of action: (a) cathodic protection, wherein the coated material serves as a sacrificial anode and polarizes the less electrochemically active metal substrate; (b) anodic passivation, wherein an ion-impermeable passivating layer is constituted on the metal surface and suppresses the oxygen reduction reaction; (c) electrolytic inhibition, wherein the corrosion cell is disrupted by blocking ion transport between the anodic and cathodic corrosion half-cells through deployment of a low-ionic-conductivity matrix; and (d) active corrosion inhibition involving the release of a sparingly soluble corrosion inhibitor upon damage to the coating, which is subsequently reprecipitated on the surface of the substrate to form a passivating layer.1,4 In recent years, there has been increasing emphasis on the design of coatings incorporating multiple modes of functionality while further keeping in mind the judicious selection of materials to minimize environmental impact.1,3,5,6 Herein, we report the development of a nanostructured magnesium/poly(ether imide) (PEI) nanocomposite system for the protection of low-alloy steels from corrosion. This novel nanocomposite system offers corrosion protection at © 2014 American Chemical Society

Special Issue: Ganapati D. Yadav Festschrift Received: Revised: Accepted: Published: 18873

March 1, 2014 May 16, 2014 June 10, 2014 June 10, 2014 dx.doi.org/10.1021/ie500897n | Ind. Eng. Chem. Res. 2014, 53, 18873−18883

Industrial & Engineering Chemistry Research

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materials.7 Fluctuations in prices of zinc have inspired an industry-wide focus on alternative coating materials that are less susceptible to disruptions in supply.8 The price and availability of zinc became a serious concern in 2006−2008, rising up to about $4200/metric ton.8 While the discovery of new mines has helped mitigate this situation, much of the steel industry is cautious about prolonged reliance on Zn as a coating material. Additionally, while Zn-rich primers are effective on steel, Zn coatings cannot be used on more electroactive metals such as lightweight Al alloys. On a more aesthetic note, Zn coatings also tend to encounter problems such as spalling (during forming) as well as blistering due to the oxidation of Zn, while further being plagued by formability and weldability issues.9,10 The incorporation of Mg in galvanized coatings, typically through physical vapor deposition or electroplating followed by subsequent annealing to induce interdiffusion of Zn and Mg, has increasingly been explored and provides significantly improved corrosion resistance.7,11−13 Hard chrome electroplating and chromate conversion coatings have also been used extensively throughout the last century owing to the ease of plating, the excellent corrosionresistant properties, and the lustrous surface finish.1,6,14,15 Selfhealing phenomena for such coatings are derived from the deposition of amorphous chromium oxide and chromium hydroxide layers through dissolution and reprecipitation mechanisms on damaged surfaces, leading to a passivated surface that precludes further corrosion. However, the potent carcinogenicity of hexavalent chromium (commonly used for both chrome electroplating and chromate coatings) has led to extremely stringent regulations with governments across the world increasingly limiting or calling for elimination of the use of chromium altogether for most civilian and military applications.16 While many alternative technologies exist, such as the use of trivalent chromium for decorative chrome electroplating, these are often unsatisfactory for corrosion resistance.17 Many alternatives have been proposed to the standard Zn, hard chrome plating, and chromate based chemistries including engineered polymers, ceramics prepared by sol−gel methods, polysiloxanes covalently bonded to hydroxyl groups on metal surfaces, self-assembled monolayers, and more complex constructs involving capsules and gels that release active corrosion inhibitors upon damage of the coating.1,15,18−20 However, most of these approaches have not translated well to industrial use owing to fundamental performance limitations. For instance, polymer coatings, while commonly used, tend to adhere poorly to metal substrates under salt water exposure due to the lack of chemical bonding and have not gained significant traction as a viable coating option.21−23 There is thus a need to design polymeric frameworks to optimize adhesion to metal surfaces. Nanostructured coatings demonstrate great potential for corrosion prevention, but issues with inevitable porosity lead to voids that result in eventual coating failure.2,24 Recent developments in the rational design of hybrid nanocomposites, wherein each component endows a specific functional property such as corrosion inhibition, formability, and adhesion to steel, have inspired a new approach to coating technologies.25−28 In this work, we demonstrate a hybrid nanocomposite coating system that combines the excellent electroactive sacrificial corrosion properties of Mg nanoplatelets with the excellent adhesion to steel and formability of a specialty polymer, PEI. This system imparts protection against corrosion through the barrier-like properties of the PEI as well as galvanic coupling of

steel to the Mg nanoplatelets; the latter, upon establishment of appropriate percolative pathways, serves as a sacrificial anode and impedes corrosion by polarizing the steel substrate. The cathodic half-reactions for corrosion of Mg and Fe can be written as follows:29 Mg 2 +(aq.) + 2e− → Mg(s); E 0 = −2.372 V vs SHE

(1)

Fe2 +(aq.) + 2e− → Fe(s); E 0 = −0.447 V vs SHE

(2)

On the basis of the stark difference in the reduction potentials, Mg is expected to polarize the steel substrate and be oxidized preferentially. The excellent sacrificial protection from corrosion that is possible with Mg has led to its use in corrosion-resistant coatings for both advanced Al alloys used in the aerospace industry as well as on steel.7,11,30−32 In terms of metallic coatings, pulsed and continuous current electrodeposition of MeMgCl precursors from tetrahydrofuran (THF) solution yields protective Mg coatings on steel, albeit with the morphologies dependent on the mode of electrodeposition. Clear indication for enhanced corrosion protection has been evidenced from potentiodynamic measurements and measurements of the open circuit potential for these electrodeposited metallic films.30,31 In an alternative approach, physical vapor deposition (electron-beam evaporation, sputtering, thermal jet vapor deposition, etc.) has been used to deposit Mg and/or Al onto galvanized steel substrates followed by a modest annealing step to promote interdiffusion of the metals.7,11−13 For the protection of Al, an entirely different approach, inspired by Zn-rich primer coatings used to protect steel,33−37 involves development of Mg-rich hybrid primers wherein Mg flakes are embedded within polymeric vehicles to provide sacrificial protection to Al metal precipitate alloys, which cannot effectively be electrochemically coupled to Zn since the former is more electroactive than Zn (in other words, Al lies lower in the galvanic series).5,38−40 In this approach, pioneered by Bierwagen and co-workers, Mg flakes that are 3070 μm in dimensions are incorporated within a variety of polymer matrices with the best results obtained for polymeric frameworks characterized by multiple cross-linking modes. The incorporated Mg flakes function best above their critical pigment volume concentration when they are in contact with each other and the substrate and can endow galvanic protection by functioning as sacrificial anodes.5,38,39 The polymer matrix mediates the kinetics of the Mg oxidation reaction in addition to serving as a barrier coating, thereby prolonging the life of the coatings. A topcoat is typically applied to these systems to prevent depletion of the Mg-rich primers and to ensure that Mg oxidation occurs only under conditions of corrosive attack.5,33,34,38,39 Despite the promising results obtained for such Mg-rich primer coatings, the large particle sizes of the incorporated Mg lead to coating thicknesses that are on the order of 70−100 μm, with the primer by itself yielding a rough surface finish. In this work, we explore an alternative strategy based on incorporation of Mg nanoplatelets within a poly(ether imide) matrix. The use of nanoplatelets with much reduced dimensions allows for high particulate concentrations to be achieved at a lower overall coating thickness (thereby providing significant weight reduction) and further allows for use of a thin topcoat since the sizes of platelets protruding from the primer layer are much lower. The polymer matrix, PEI, examined in this work has also not hitherto been examined within Mg-rich primers and is shown to exhibit remarkable performance 18874

dx.doi.org/10.1021/ie500897n | Ind. Eng. Chem. Res. 2014, 53, 18873−18883


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the reaction had an estimated molecular weight of ∼120 000 g/ mol.9 To prepare the precursor formulations for nanocomposite coatings, the Mg nanoplatelets were introduced to the asprepared PAA solution using a homogenizer while maintaining the appropriate viscosity with the addition of NMP. The Mg nanoplatelet-containing primer was formulated with particle loadings of 10, 20, and 40 wt % Mg in PAA. In a typical formulation step for preparing 10 wt % Mg/PAA, 375 mg of Mg was sonicated in 2 mL of NMP for 10 min, followed by the addition of 15 mL of 25 wt % PAA. The mixture was then homogenized for 5−10 min, yielding the 10 wt % Mg/PAA precursor solution for application onto steel. Zn particles (800 nm) were purchased from U.S. Research Nanomaterials Inc. and introduced to the PAA in the same manner as the Mg nanoplatelets. Application of Coatings. The nanocomposite coatings were deposited by dip coating the Mg-rich primer layer using a TLO.01 Dip coater (MTI Corporation). The clean low-alloy steel substrates were immersed in the solution for 1 min before withdrawing at a speed of 140 mm/min, followed by spray coating of the PEI topcoat layer using a Master airbrush with a nozzle diameter of 0.5 mm and an air compressor with an output pressure of 45 psi. The steel substrate had a thickness of 0.67 mm and nominal impurity content of 0.1% C, 1.5% Mn, 0.2% Si, 40 ppm of N, 0.01% P, 0.002% S, and 0.003% Ti. The steel was cleaned with hexanes to remove a temporary anticorrosive coating of grease, followed by ethanol and detergent (Alconox), and finally rinsed with deionized water prior to application of the coating. PAA was applied to the steel and cured at 150 °C for 5 min, followed by a 250 °C curing step for 5 min to complete the imidization of the PEI (as per Scheme 1) and remove residual NMP. Average dry thicknesses of the primer and topcoat were 12 and 18 μm, respectively, as determined using a byko-test 8500 thickness gauge (BYK). The structure of PEI was confirmed by FTIR (Bruker Vertex 70), and the surface properties of the coatings were evaluated by water contact angle measurements using glass-distilled water showing contact angles of 113° ± 1° on a reference foil of pure polytetrafluoroethylene (Carborundum Type C Teflon). The water was transferred dropwise onto the coating surface on the stage of a Rame′-Hart NRL 100A Contact Angle Goniometer, using a freshly flamed platinum wire. The test droplets were observed through a simple microscope at 10× magnification to measure the contact angle. Adhesion Testing. Assessment of the adhesion of the coatings to the low-alloy steel substrate was performed using American Society for Testing of Materials (ASTM) Method D3359. The cross-cuts were made in the coatings using a BYK Cross-Cut-Tester with 1 mm spacing between blades. Ratings were given to each coating as per the ASTM protocol with a classification ranging from 5B being regarded as perfect (where the coating is not visibly removed) to 0B (corresponding to removal of greater than 65% of the coating). Morphological Characterization. The morphological features of the Mg nanoplatelets and the as-prepared composite coatings were examined by scanning electron microscopy (SEM, Hitachi SU-70 operated at an accelerating voltage of 20 kV). The Mg nanoplatelets were also examined by transmission electron microscopy (TEM, JEOL-2010, accelerating voltage of 200 kV, 100 mA) on 400 mesh Cu grids. To study the crosssections of the composite coatings and to examine the dispersion of the nanoplatelets within the polymer matrix, the

derived from its excellent flexibility, formability, thermal stability, chemical bonding to metal surfaces, and a high glass transition temperature of 155 °C.9,27,41 In past work, we have developed a scalable and atom-efficient route to prepare airstable Mg nanoplatelets by the solution-phase reduction of Grignard’s reagents.42 In this article, we have examined the corrosion inhibition of low-alloy steel substrates endowed upon coating with a primer containing three different loadings of the Mg nanoplatelets: 10, 20, and 40 wt % of the nanoplatelets dispersed in PEI followed by a spray-deposited PEI overlayer. These Mg/PEI coatings have been compared to galvanized steel, uncoated low-alloy steel, PEI alone, as well as PEI coatings incorporating Zn nanoflakes at the same three concentrations.

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METHODS Synthesis of Mg Nanoplatelets. Mg nanoplatelets were synthesized by the solution-phase reduction of MeMgCl using lithium naphthalide in anhydrous THF.42 The Mg product was collected by centrifugation, washed with THF several times, and subsequently dried under vacuum conditions. The product was characterized by X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5418 Å). Synthesis of Polyamic Acid (PAA) and Composite Formulations. PEI was synthesized via the polymerization of a PAA intermediate (Scheme 1). PAA was prepared by the Scheme 1. Reaction Scheme for the Synthesis of PEI on Steel

copolymerization of 4,4′-biphthalic anhydride (Sigma-Aldrich) with two separate diamines: m-phenylenediamine and 1,2-bis(2aminoethoxyethane) (both from Sigma-Aldrich) in N-methylpyrrolidone (NMP; Scheme 1). In a typical synthesis, a reaction vessel was charged with the following reactants: 25 mL of NMP as the solvent, 4.3 g of 4,4′-biphthalic anhydride, 0.5 g of mphenylenediamine, and 1.5 g of 1,2-bis(2-aminoethoxyethane), and the mixture was stirred for 10−16 h at room temperature, causing it to become increasingly viscous with progress of the polymerization reaction; the PAA obtained at the conclusion of 18875



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fracture surfaces were imaged by SEM after cryo-fracturing freestanding polymer composites and coating the fracture surfaces with a thin layer of conductive amorphous carbon to mitigate the effects of charging. Secondary electron images as well as backscattered electron images were collected along with energy dispersive X-ray (EDX) spectra for the confirmation of the location of the Mg nanoplatelets within the polymer matrix. Accelerated Corrosion Testing. Electrochemical characteristics of the coatings were evaluated at a scan rate of 1 mV/s using potentiodynamic measurements (ASTM G59) performed on a Princeton Applied Research 263A potentiostat (PowerSuite electrochemistry software) with an aqueous solution of 3.5% NaCl as the electrolyte. A standard calomel electrode and Pt strip were used as the reference and counter electrodes, respectively. All measurements were performed after the system had reached the “steady state” as indicated by a stable open circuit potential. The corrosion rate was extrapolated using the ASTM G59 standard test method for conducting potentiodynamic polarization resistance measurements as well as ASTM G102 standard practice for calculation of corrosion rates and related information from electrochemical measurements. Coatings were also exposed to a 3.5% aqueous solution of NaCl for an extended period of time to monitor the corrosion of the steel. Weight loss experiments were performed using a cell designed to only test the coated surface rather than the entire steel sheet, thereby avoiding contributions from uncoated regions or sample edges to the test data. The cell was constructed from an acrylic block; a hole was drilled through the middle, and a seal with the coated face of the steel substrate was made using an O ring. Clamps were placed on the acrylic block, and a 3.5% aqueous solution of NaCl was added to the cylindrical channel, making contact with the coated low-alloy steel substrate for the duration of the test period. The volume of the solution was kept constant for the duration of the test. Once the test was deemed complete, each specimen was cleaned using a solution of 0.7 g of hexamethylenetetramine, 100 mL of hydrochloric acid, and 100 mL of water as per ASTM G1 standards. Subsequent to the removal of rust, the surface was quickly washed with water, dried, and weighed immediately to avoid any further corrosion. The corrosion rate was extrapolated using the ASTM G1 standard practice for preparing, cleaning, and evaluating corrosion test specimens. Additionally, coatings were scratched down to the steel surface and submerged in a 3.5% NaCl solution for 1 week to examine the protection of the steel surface by the sacrificial oxidation of the Mg from the Mg-rich primer layer.

Figure 1. Powder XRD pattern of Mg nanoplatelets matched to hexagonal Mg (JCPDS 35-0821).

Figure 2. (A, B) SEM and (C, D) TEM images of Mg nanoplatelets prepared by the reduction of MeMgCl by lithium naphthalide.

using a homogenizer prior to mechanical mixing ensures the stabilization of highly concentrated dispersions of Mg in PAA. Figure 3 depicts SEM images of the nanocomposite coatings incorporating Mg nanoplatelets (vide inf ra). PAA has been synthesized by copolymerization of an anhydride and two diamines in NMP as depicted in Scheme 1. The Mg nanoplatelets are introduced to a small amount of NMP and sonicated before mixing with the PAA using a homogenizer for 5−10 min. The stable, viscous Mg/PAA solution is then dip-coated onto a cleaned and degreased lowalloy steel substrate, and the PEI is generated in situ on the steel surface through the imidization reaction depicted in Scheme 1, thereby fashioning a Mg-rich primer layer with a thickness of ca. 15 μm. In situ imidization has been observed to substantially enhance the adhesion of the polymer to the steel surface, yielding a mechanically robust composite.9,27,28,43 The mphenylenediamine component has been intentionally introduced to the reaction mixture to increase disorder, impede crystallization of the polymer, and thus enhance flexibility and formability as the coating wraps around the Mg nanoplatelets and adopts the contours of the low-alloy steel surface.9

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RESULTS AND DISCUSSION Mg nanoplatelets have been synthesized for use in the PEI nanocomposites (see Methods) using the solution-phase reduction of MeMgCl by lithium naphthalide and are characterized by XRD, SEM, and TEM as shown in Figures 1 and 2.42 The indexed powder XRD pattern (Figure 1) matches Joint Committee on Powder Diffraction Standards (JCPDS) Card #35-0821 for hexagonal Mg and confirms that the obtained products are phase pure. SEM images of the Mg nanoplatelets (Figure 2A and B) depict extensive agglomeration; however, the roughly hexagonal platelet shape of the platelets can be discerned. This morphology is further verified by TEM images shown in Figure 2C and D along with a more detailed analysis of size distribution in our previous work,42 which indicates that the Mg nanoplatelets range in size from 100 to 500 nm. Ultrasonication of the nanoplatelets in NMP 18876



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Mg nanoplatelets (Figure 3C, E, G). In addition, backscattered electron images display the atomic-number contrast (Zcontrast) between the Mg and carbon from the PEI matrix allowing for further confirmation that the observed features indeed correspond to Mg nanoplatelets (Figure 3D, F, H). Figure 3B provides verification that the PEI alone does not display such atomic-number contrast. Good dispersion and the absence of phase segregation of the metallic fillers is noted even for the sample with 40 wt % loading of the Mg nanoplatelets. Quality dispersion of the filler material in a nanocomposite is key to establishment of a percolative network, which is crucial for galvanic coupling to the underlying steel substrate.5,28 Limiting phase segregation is also important to limit the porosity of the coatings and to ensure mechanical integrity under accelerated testing conditions. As further evidence that the contrast in the backscattered images and morphological features noted in the secondary electron images indeed arise from the incorporation of Mg nanoplatelets, EDX spectra have been acquired for two different regions of a 40 wt % Mg/PEI sample (Figure S3, Supporting Information). When a spot is selected for analysis on a brighter region of a backscattered SEM image, the EDX signal for Mg is approximately 4 times as intense as carbon from the PEI (Figure S3A). In contrast, Figure S3B shows EDX spectra for a spot in a darker area in the backscattered SEM image where the signal for carbon is much greater than that of Mg, verifying the assignment of the contrast mechanism in the backscattered images as arising from the higher electron density of Mg as compared to carbon. The adhesion of the coatings to the steel surface is of paramount importance in any application where barrier properties are essential; thus, the adhesion of the coatings has been tested using ASTM D3359, which involves peeling of a tape and observing the extent to which the sample is delaminated. Each coating sample has been accorded a rating based on the test specifications where 5B corresponds to excellent adhesion (0% of the coating is removed) and 0B corresponds to removal of more than 65% of the coating by the tape. All of the coatings tested received a rating of 5B, indicating that the PEI alone shows excellent adhesion to lowalloy steel and that the incorporation of Mg nanoplatelets does not deleteriously impact the adhesion even for 40 wt % particle loadings (the left column of Figure 4A−H). It is likely that hydroxyl groups on the cleaned steel surfaces can condense with the secondary amines in the PAA framework, thus covalently linking the polymeric matrix to the steel surface. Indeed, our past experience with spin-coating PEI directly onto the steel substrates indicates much worse adhesion and blistering/delamination of the films upon extended immersion in salt water. As a qualitative measure of the sacrificial protection afforded by the coatings to the different test substrates, cross-cuts have been made in all of the coatings, down to the steel substrate, followed by exposure of this same area (each cut being 2 cm in length) to 3.5% aqueous solution of NaCl for 1 week. As noted above, the premise of the galvanically coupled Mg-rich primer is that when the coating is damaged, the Mg nanoplatelets ought to be preferentially oxidized, and the oxidation products should occupy the incipient cracks and voids that are created. Figure 4 shows digital photographs of the panels after adhesion testing (left column of Figure 4A−H), whereas the right panels (Figure 4A−H) exhibit the same test panels after exposure to a saline environment for 1 week. Figure 4A shows clear evidence for red

Figure 3. Cryo-fractured SEM of (A, B) PEI, (C, D) 10 wt % Mg/PEI, (E, F) 20 wt % Mg/PEI, and (G, H) 40 wt % Mg/PEI. Secondary electron images are shown in A, C, G, and E; backscattered electron images are shown in B, D, F, and H.

FTIR spectroscopy has been performed to verify the structure of the PEI (Figure S1, Supporting Information). Figure S1 indicates imide carbonyl stretching frequencies at 1780 and 1713 cm−1, as well as C−N stretching modes for the imide linkage at 1362 cm−1; these modes are characteristic of the five-membered ring structure of PEI.27 Subsequent deposition of PAA to create a topcoat layer of PEI via spray coating seals in the Mg-rich primer layer and completes the coating system with an overall dry film thickness of ca. 30 μm. Water contact angle measurements have been performed on the composite coatings to determine their hydrophilicity/ hydrophobicity, and a contact angle of ca. 70° is measured. Although not entirely hydrophobic, the nanocomposite coatings provide good barrier properties for corrosion protection. SEM has been used to examine the microstructure of the composite coatings and suggests that the coatings are relatively featureless with a smooth surface; no cracks or pinhole voids are discernible, which is critical for the barrier functions of the coating (Figure S2, Supporting Information). Due to the ca. 18 μm topcoat of PEI, the Mg nanoplatelets are not observable by SEM in the top-view images. The dispersion of the Mg in the PEI matrix has been examined using cross-sectional SEM of cryo-fractured surfaces of free-standing composite samples (Figure 3). Figure 3A shows that cross-sectional images of PEI are relatively featureless with no significant morphological features, whereas cross-sectional micrographs of the nanocomposite coatings clearly indicate the presence of protruding 18877



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Figure 4. Digital photographs of (A) PEI, (B) 10 wt % Mg/PEI, (C) 20 wt % Mg/PEI, (D) 40 wt % Mg/PEI, (E) PEI on galvanized steel, (F) 10 wt % Zn/PEI, (G) 20 wt % Zn/PEI, and (H) 40 wt % Zn/PEI coatings on low-alloy steel, with the exception of E on galvanized steel. The image in the left column of each figure shows the sample after making cross-cuts and performing adhesion testing; the image in the right column of each figure shows the samples after exposure to 3.5% aqueous solution of NaCl for 1 week. The relatively enhanced protection against red rust formation is seen to correlate with the amount of Mg nanoplatelets incorporated in the coating, while incorporation of Zn particles at the same concentrations does not seem to afford additional corrosion protection. Each sample had an area with a diameter of 2 cm exposed to the salt water solution (circled area) and the distance between each cut is 1 mm.

rust formation, suggesting that the PEI coating alone when scratched does not provide sufficient protection to the steel surface to preclude rust formation. In stark contrast, Figure 4B−D show the relatively enhanced protection of the steel surface through the preferential oxidation of the Mg nanoplatelets from the Mg-rich primer layer. The prediliction for red rust formation appears to decrease with increasing concentration of incorporated Mg nanoplatelets with the 40 wt % Mg/ PEI sample showing almost entirely white corrosion products derived from oxidation of Mg. The corrosion products appear to be amorphous by powder X-ray diffraction (not shown). Raman spectroscopy (data not shown) yields a broad spectrum without any evidence for well-defined peaks that could be assigned to crystalline species. A mix of amorphous oxides,

hydroxides, and carbonates is expected to be stabilized and, given their low Ksp values, may further inhibit cathodic reactions on the substrate beyond the galvanic protection provided by oxidation of Mg.38 Indeed, past work by Bierwagen has indicated the initial stabilization of Mg(OH)2, which subsequently reacts with CO2 from the ambient atmosphere to form MgCO3.5 Dissimilarly, Zn/PEI composite coatings prepared at 10, 20, and 40 wt % show little to no sacrificial protection of the underlying steel substrate (Figure 4F−H), likely due to the lack of a percolative network on account of a much lower concentration of metal filler which has been used in this work compared to other Zn-rich primer technologies. Additionaly, Figure 4E displays a PEI coating on galvanized 18878



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wt %, respectively) is introduced by incorporation of Mg nanoplatelets within the PEI coating, suggesting a clear role for sacrificial cathodic protection.30,31 The total polarization of 240−280 mV renders the oxidation of Fe in steel much more difficult. Note that the shift in corrosion potential is less than that of metallic Zn owing to the distributed Mg matrix formed for the system under consideration here, which is quite distinct from a continuous metallic Mg coating. Interestingly, under analogous particle loadings, Zn/PEI coatings constituted from ca. 800 nm Zn nanoparticles only show a modest additional polarization of 40 mV, suggesting that the Zn primers do not provide as high a degree of sacrificial cathodic protection as the Mg nanoplatelets at the same primer loadings (perhaps due to absence of good percolation). The anodic branch of the polarization curves measured for the Mg/PEI samples shows several distinct regimes: the initial regime from their respective corrosion potentials up to −0.43 V corresponds to primarily Mg oxidation without oxidation of the Fe substrate.39 Only with further anodic polarization above −0.43 V is there an increase in the slope of the curve corresponding to a breakdown of the coating most likely due to pitting corrosion.39 In contrast to steel and galvanized steel, the anodic current of all Mg containing coatings levels off at around 0 V versus SCE, indicating mass transfer limited dissolution. In contrast, Zn/PEI coatings display a much smaller range of passivation (from their respective corrosion potentials to about −0.62 V) followed by a sharp increase in the slope due to breakdown of a putative oxide layer. The potential at which corrosion of the substrate is initiated is significantly more anodic, and the range of the passivation plateau is substantially expanded upon incorporation of the Mg nanoplatelets in the PEI. The anodic (as well as the cathodic) current density in these samples is well correlated to (and monotonically decreases with) the amount of Mg nanoplatelets incorporated within the coatings likely due to decreasing porosity, better interparticle contact, and good electrical percolation.40 An analogous, but less pronounced, trend is observed with increased loading of Zn nanoplatelets in PEI (Figure S4, Supporting Information and Table 1). As illustrated in Table 1, for the coating with 40 wt % Mg, the corrosion current density drops to a remarkably low value of 3.61 A cm−2, decreasing by over 5 orders of magnitude as compared to bare low-alloy steel (about 3 orders of magnitude as compared to using just PEI as the coating and about 2 orders of magnitude compared to using 40 wt % Zn/PEI as the coating).

steel and the preferential oxidation of the Zn for the protection of the steel substrate. Potentiodynamic electrochemical measurements have been performed on the coated samples to further examine the ability of the coatings to resist corrosion in saline environments. The Mg/PEI nanocomposite coatings have been compared to galvanized steel, bare low-alloy steel, PEI alone, and Zn/PEI coatings at the same three concentrations (10, 20, and 40 wt %) to garner an understanding of the degree of enhanced corrosion protection endowed by the nanocomposite coatings. Figure 5

Figure 5. Potentiodynamic plots showing the relatively enhanced corrosion resistance afforded by the composite coatings as compared to the behavior of the PEI coating, 40 wt % Zn/PEI, uncoated steel, and galvanized steel. All of the PEI and composite coated samples have a topcoat of PEI.

displays a Tafel plot of current density versus potential acquired in a 3.5% aqueous solution of NaCl. Additional data are included in the Supporting Information (Figure S4) corresponding to 10 and 20 wt % Zn/PEI in addition to the samples shown in Figure 5. A first observation is that the PEI coating alone brings about an approximately 2-orders-of-magnitude diminution in the corrosion current density for both anodic and cathodic branches as compared to steel and galvanized steel, resulting in a projected corrosion rate (mm/y) that is 3-ordersof-magnitude less than that of the bare low-alloy steel (Table 1). Next, we note that the corrosion potential of the PEI-coated steel sample is shifted from −640 mV (for bare steel) to −780 mV, suggesting a 140 mV polarization of the steel simply upon application of the polymeric coating. An additional 140 or 100 mV polarization (to −0.92 and −0.88 V for 10 wt % and 20/40

Table 1. Corrosion Potential (Ecorr), Corrosion Current Density (Icorr), and Extrapolated Corrosion Rate in mm/y from Electrochemical and Weight-Loss Tests, for Galvanized Steel, Uncoated Low-Alloy Steel, PEI Coating, Zn/PEI Coatings, and the Mg Nanocomposite Coatings sample galvanized steel uncoated low-alloy steel PEI coating 10 wt % Zn/PEI 20 wt % Zn/PEI 40 wt % Zn/PEI 10 wt % Mg/PEI 20 wt % Mg/PEI 40 wt % Mg/PEI

ECorr (V)

ICorr (A/cm2)

electrochemical corrosion rate (mm/y)

weight-loss (mg)

weight-loss corrosion rate (mm/y)

Zn Fe

−1.05 −0.64

2.24 × 10−6 9.27 × 10−6

2.60 × 10−2 1.08 × 10−1

54.6 165.0

3.37 × 10−2 ± 0.2% 1.02 × 10−1 ± 0.2%

Fe Zn/Fe Zn/Fe Zn/Fe Mg/Fe Mg/Fe Mg/Fe

−0.78 −0.82 −0.82 −0.82 −0.92 −0.88 −0.88

5.86 5.00 3.72 3.41 2.23 7.91 3.61

10−4 10−5 10−5 10−5 10−4 10−6 10−7

12.5

7.73 × 10−3 ± 1.2%

7.4 1.1 0.1

4.57 × 10−3 ± 1.5% 6.80 × 10−4 ± 0.6% 6.18 × 10−5 ± 1.8%

metal dissolution contributing to corrosion current

× × × × × × ×

10−8 10−9 10−9 10−9 10−8 10−10 10−11

18879

6.80 5.81 4.32 3.95 2.59 9.18 4.19

× × × × × × ×



Article

The projected corrosion rate (CR) in millimeters per year has been extrapolated from the Tafel plots based on the expression: CR =

Icorr ·κ ·EW ρ

(3)

where Icorr is the corrosion current density, κ is a constant of 3272 mm g/A cm y used to obtain the desired units of mm/y, EW is the equivalent weight (27.92 g for steel, as specified by ASTM G59), and ρ is the density (7.87 g/cm3 for steel). The extrapolated corrosion rate values for the samples deduced using this expression are listed in Table 1. For all of the nanocomposite coatings, the dissolution of Mg cannot be ignored (given the cathodic mode of action); therefore the values calculated are measured galvanic corrosion rates. In other words, a major caveat to developing estimates of corrosion rate for the nanocomposite coatings is that the Fe/Mg couple is being simultaneously measured. Also, given the distributed nature of the Mg nanoplatelets, it is difficult to rigorously estimate the geometric area of the Mg primer particles. Keeping these caveats in mind, it is still useful to develop a semiquantitative measure of the extent of corrosion protection. Compared to the uncoated low-alloy steel, the 40 wt % Mg/PEI coating yields almost a 6 orders of magnitude diminution in corrosion rate. While the 10 wt % Mg/PEI coating shows only minor enhancement in corrosion protection over the PEI alone, the 20 and 40 wt % Mg/PEI nanocomposite coatings show remarkable reduction in corrosion rate by approximately 2 and 3 orders of magnitude, respectively, as compared to the PEI coating. The Zn/PEI coatings show a reduction in corrosion rate of approximately 1 order of magnitude versus PEI, whereas the 20 and 40 wt % Mg/PEI coatings show a 1 and 2 orders decrease in corrosion rate, respectively, as compared to 40 wt % Zn/PEI. The cathodic branch of the polarization curve for the bare low alloy steel corresponds to the diffusion limited reduction of oxygen with a relatively well-defined plateau followed by a higher slope likely due to reduction of protons. The cathodic branches of the plots for the three different Mg loadings run parallel to each other, indicating similar behavior for all three samples. The higher slopes and lack of a well-defined plateau evidenced in the cathodic polarization curves for the Mg/PEIcoated samples suggest simultaneous occurrence of other reactions such as water reduction along with oxygen reduction. Battocchi et al. have shown that pure Mg oxidation occurs through mixed cathodic−anodic control, whereas in this case the Mg nanoplatelets in the PEI matrix exhibit anodic control of the oxidation; the PEI matrix substantially slows down the kinetics of the oxidation of the Mg, prolonging its effective lifetime within the coating.39 Additional evaluation of the corrosion protection provided by the different nanocomposite coatings has been performed by immersing the test panels in a 3.5% aqueous solution of NaCl for an extended period of time. Figure 6 displays digital photographs of samples exposed to the saline environment for different durations: 0, 144, 1200, and 1872 h, where the final time marks the end of the test. As evidenced by the images shown in the second column of Figure 6, after just 144 h exposure to the NaCl solution, extensive red rust formation is seen on the low-alloy steel surface, whereas the galvanized steel sample is characterized by white deposits suggesting the sacrificial oxidation of the Zn overlayer (likely to ZnO and simonkolleite Zn5(OH)8Cl2·H2O).7 All three nanocomposite

Figure 6. Digital photographs of salt-water immersion measurements on (A) galvanized steel, (B) uncoated low-alloy steel, (C) low-alloy steel with PEI coating, (D) low-alloy steel with 10 wt % Mg/PEI coating, (E) low-alloy steel with 20 wt % Mg/PEI coating, and (F) low-alloy steel with 40 wt % Mg/PEI coating. Each sample has an area with a diameter of 3.5 cm exposed to the 3.5% solution of NaCl in water.

coatings as well as the substrate coated with PEI alone show no signs of breakdown or corrosion after this period of time. The third column in Figure 6 demonstrates that after 1200 h of exposure to the 3.5% NaCl solution, further breakdown of the Zn overlayer occurs, and red rust formation is initiated for the galvanized steel sample as the galvanically coupled zinc is entirely consumed. In contrast, the low-alloy steel sample at this point is almost entirely covered with red rust. The PEI coating shows some signs of minor delamination as well as a few areas where red rust formation has been initiated. At this interval, the Mg/PEI nanocomposite coatings do not exhibit comparable signs of failure. The 10 wt % Mg/PEI coating shows some traces of red rust formation, whereas the 20 wt % Mg/PEI coating shows some white oxidation products, likely 18880



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composite coating with potentiodynamic polarization, the outer environment can differ from the local corrosion environment at the surface of the steel substrate, which gives rise to a scenario distinct from metallic coatings.44 In the same vein, weight-loss measurements for polymeric coatings can also be complicated due to the required cleaning step, which can introduce an error referred to as the “cleaning factor.”46 The error associated with the cleaning factor is usually on the order of 1−2% but can be somewhat more involved for polymeric coatings owing to the possibility of the breakdown and loss of the coating during cleaning or the entrapment of corrosion products under the polymer coating.44,46,47 Much of this error has been addressed in our work by the evaluation of multiples of the samples (each sample was tested in triplicate), the use of prolonged exposure, and adherence to the guidelines provided in the standardized protocols. Despite the caveats related to the extrapolated lifetimes, taken together, both the weight loss data and electrochemical characterization are consistent with a general trend of increasing volume concentration of incorporated Mg nanoplatelets leading to greater corrosion protection. The role of the PEI polymer is worthy of discussion since it is clear from comparison to results for galvanized steel (Figures 5 and 6) that the nanocomposite coatings offer more than simply sacrificial cathodic protection. As noted above, the strong adhesion of PEI likely results from covalent linkages to hydroxyl groups on the metal surfaces. The highly cross-linked PEI matrix mediates the kinetics of the Mg oxidation reaction and itself provides a barrier to electrolyte transport.

from sacrificial Mg oxidation. The 40 wt % Mg/PEI coating does not show any indication of being compromised after 1200 h. Finally, after completion of the immersion test at 1872 h, widespread corrosion and red rust formation is evidenced across all of the test substrates except for the 20 and 40 wt % Mg/PEI coatings. The 20 wt % Mg/PEI coating shows only slight red rust formation, with a much greater extent of white corrosion products derived from sacrificial oxidation of Mg. The Mg corrosion products are more soluble than their Zn analogues and can traverse through the coating and be precipitated on the surface. For the 40 wt % Mg/PEI coating, some specks of red rust are apparent, but the 20 and 40 wt % Mg/PEI coatings are clearly superior to PEI alone or even galvanized Zn in protecting the low-alloy steel substrates from corrosion. In addition, Figure S5 (Supporting Information) shows a magnified view of each sample after conclusion of the weight-loss test providing better visualization of red rust formation. In order to acquire a more quantitative understanding of the corrosion protection provided by the nanocomposite coatings, weight-loss experiments have been performed for the same samples depicted in Figure 6. The weight-loss measurements are standardized by ASTM G1 as described in the Methods section. This standardized protocol allows for the evaluation of the corrosion rate (CR) in millimeters per year based on the expression: ⎛ W ⎞ ⎟ CR = 87.6⎜ ⎝ DAT ⎠

■

(4)

CONCLUSIONS In summary, we have developed a novel hybrid nanocomposite system for enhanced corrosion protection of low-alloy steels that appears to function through active−passive modalities involving barrier action to corrodants, sacrificial cathodic protection, as well as formation of ion-impermeable passivating layers. The coating systems are formulated by homogeneously dispersing Mg nanoplatelets within PAA matrices (as verified by cross-sectional SEM images); the Mg nanoplatelets are prepared by the lithium naphthalide reduction of Grignard’s reagents and have been characterized by powder X-ray diffraction, SEM, and TEM. The blended composites are further imidized after application to low-alloy steel substrates to yield Mg/PEI nanocomposite coatings (as corroborated by FTIR measurements). The hybrid Mg/PEI nanocomposites combine the excellent formability and adhesion to steel of the polymer matrix with the sacrificial cathodic protection of the Mg nanoplatelets, providing excellent corrosion resistance at a significantly reduced weight as compared to galvanized Zn technologies of comparable thickness as well as Zn/PEI coatings at the same concentrations. To the best of our knowledge, this is the first time that Mg nanoparticles have been incorporated as fillers within coatings for corrosion inhibition; prior work has focused on micrometer-sized particles that are at least 2 and sometimes 3 orders of magnitude greater in size. The use of nanoplatelets further allows for high particulate concentrations to be achieved at a lower overall coating thickness as well as requiring a lower topcoat thickness to completely cover the Mg filler material (due to its smaller size) providing for a reduced weight penalty. The Mg nanoplatelets are observed to synergistically couple to the PEI matrix, which is prepared by the in situ imidization of PAA. Scratch testing of the nanocomposite coatings evidences the preferential oxidation of the Mg nanoplatelets over the steel

where 87.6 is a constant used to obtain the desired units, W is the weight loss in milligrams, D is the density of the metal in g/ cm3, A is the surface area exposed to the NaCl solution in cm2, and T is the duration of exposure to the NaCl solution in hours. Corrosion rate values for the samples are listed in the last column of Table 1 (based on samples measured in triplicate). Compared to bare low-alloy steel, the 40 wt % Mg/PEI nanocomposite coating yields an approximately 4 orders of magnitude diminution in corrosion rate. The 10 wt % Mg/PEI coating provides slightly better corrosion protection than the PEI alone, likely since at this particle loading a complete percolative network has not been established, and furthermore there is only a limited concentration of Mg available to buffer a corrosive attack. In contrast, 20 and 40 wt % Mg/PEI coatings exhibit approximately a 1 and 2 orders of magnitude decrease in corrosion rate, respectively, as compared to the PEI coating. Differences in the estimated corrosion rates between the electrochemical characterization and weight-loss test are apparent. It is not uncommon for weight-loss tests to provide extrapolated corrosion rates that are higher than those deduced from Tafel analysis.44,45 The electrochemical characterization generates a snapshot in time of the corrosion occurring at the steel surface, and the corrosion rate is extrapolated assuming constancy of the corrosion current; in contrast, the weight-loss test captures degradation over a prolonged period and the gradual breakdown of the coating, leading to an average corrosion rate across the sample surface for the measurement period.44 The weight-loss measurement is also more susceptible to pinholes resulting from localized catastrophic failure in the coating, which can deleteriously alter the extrapolated corrosion rate. It is also worth noting that there are a number of caveats to using weight-loss measurements and potentiodynamic polarization analysis to extrapolate the corrosion rates and lifetimes of polymeric coatings. When probing the surface of a 18881



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(8) Zinc Monthly Price - US Dollars per Metric Ton. http://www. indexmundi.com/commodities/?commodity=zinc&months=360 (accessed May 10, 2014). (9) Rout, T. K.; Gaikwad, A. V.; Dingemans, T. A Method of Preparing a Polyetherimide Coating on a Metallic Substrate. World Intellectual Property Organization. Patent number WO 2011035920 A1, 2011. (10) Marder, A. R. The Metallurgy of Zinc-Coated Steel. Prog. Mater. Sci. 2000, 45, 191−271. (11) Prosek, T.; Nazarov, A.; Bexell, U.; Thierry, D.; Serak, J. Corrosion Mechanism of Model Zinc−magnesium Alloys in Atmospheric Conditions. Corros. Sci. 2008, 50, 2216−2231. (12) Volovitch, P.; Allely, C.; Ogle, K. Understanding Corrosion via Corrosion Product Characterization: I. Case Study of the Role of Mg Alloying in Zn−Mg Coating on Steel. Corros. Sci. 2009, 51, 1251− 1262. (13) Volovitch, P.; Vu, T. N.; Allély, C.; Abdel Aal, A.; Ogle, K. Understanding Corrosion via Corrosion Product Characterization: II. Role of Alloying Elements in Improving the Corrosion Resistance of Zn−Al−Mg Coatings on Steel. Corros. Sci. 2011, 53, 2437−2445. (14) Groshart, E. Finishing in the Green: Chromium Plating Replacements. Met. Finish. 1997, 95, 70−72. (15) Bibber, J. W. Chromium-Free Conversion Coatings for Zinc and Its Alloys. J. Appl. Surf. Finish. 2007, 2, 273−275. (16) Agency for Toxic Substances and Disease Registry; Department of Health and Human Services: Atlanta, GA, 1998. (17) Pollution Prevention Technology Profile Trivalent Chromium Replacements for Hexavalent Chromium Plating; Northeast Waste Management Officials’ Association: Boston, MA, 2003. (18) Rout, T. K.; Jha, G.; Singh, A. K.; Bandyopadhyay, N.; Mohanty, O. N. Development of Conducting Polyaniline Coating: A Novel Approach to Superior Corrosion Resistance. Surf. Coat. Technol. 2003, 167, 16−24. (19) Choy, K. L. Chemical Vapour Deposition of Coatings. Prog. Mater. Sci. 2003, 48, 57−170. (20) Zheludkevich, M. L.; Salvado, I. M.; Ferreira, M. G. S. Sol-Gel Coatings for Corrosion Protection of Metals. J. Mater. Chem. 2005, 15, 5099−5111. (21) Bellucci, F.; Nicodemo, L.; Monetta, T.; Kloppers, M. J.; Latanision, R. M. A Study of Corrosion Initiation on Polyimide Coatings. Corros. Sci. 1992, 33, 1203−1226. (22) Schilling, P. J.; Herrington, P. D.; Daigle, E. O.; Meletis, E. I. Surface Characteristics of Structural Steel Processed Using ElectroPlasma Techniques. J. Mater. Eng. Perform. 2002, 11, 26−31. (23) Roy, D.; Simon, G. P.; Forsyth, M.; Mardel, J. Towards a Better Understanding of the Cathodic Disbondment Performance of Polyethylene Coatings on Steel. Adv. Polym. Technol. 2002, 21, 44−58. (24) Erb, U.; Palumbo, G.; Zugic, R.; Aust, K. T. Processing and Properties of Nanocrystalline Materials. In Processing and Properties of Nanocrystalline Materials; Suryanarayana, C.; Singh, J.; Froes, F. H., Eds.; TMS: Warrendale, PA, 1995. (25) Crosby, A. J.; Lee, J. Polymer Nanocomposites: The “Nano” Effect on Mechanical Properties. Polym. Rev. 2007, 47, 217−229. (26) Noor, A. K.; Venneri, S. L. Flight-Vehicle Materials, Structures, and Dynamics: Advanced Metallics, Metal-Matrix and Polymer-Matrix Composites; Flight-vehicle Materials, Structures, and Dynamics: Assessment and Future Directions; American Society of Mechanical Engineers: New York, 1994. (27) Rout, T. K.; Gaikwad, A. V.; Lee, V.; Banerjee, S. Hybrid Nanocomposite Coatings for Corrosion Protection of Low Carbon Steel: A Substrate-Integrated and Scalable Active−passive Approach. J. Mater. Res. 2011, 26, 837−844. (28) Dennis, R. V.; Viyannalage, L. T.; Gaikwad, A. V.; Rout, T. K.; Banerjee, S. Graphene Nanocomposite Coatings for Protecting LowAlloy Steels from Corrosion. Am. Ceram. Soc. Bull. 2013, 92, 18−24. (29) CRC Handbook of Chemistry and Physics, 94th ed.; Haynes, W. M., Ed.; CRC Press: Boca Raton, FL, 2013; Section 5, pp 81−82. (30) Ben Hassen, S.; Bousselmi, L.; Rezrazi, E. M.; Berçot, P.; Triki, E. Comparative Study of Protective Magnesium Deposit Behaviour

substrate with the formation of amorphous corrosion products. Potentiodynamic testing of the samples as well as salt water immersion tests indicate a significant enhancement in corrosion protection with the addition of the Mg nanoplatelet filler. The Mg-rich primer layer with a PEI topcoat represents a scalable solution for the replacement of hexavalent chromium and an alternative to galvanized Zn protection of steel with an eye on the bright future of lightweight metal composites.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1, FTIR of PEI; Figure S2, SEM microstructure of the top surface of the Mg/PEI composite coating; Figure S3, EDX spectrum of the 40 wt % Mg/PEI cryofractured coating; Figure S4, Tafel plot displaying a complete comparison of all coatings; and Figure S5, a detailed view (digital photographs) of each sample after weight-loss testing had concluded. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: sb244@buffalo.edu. Present Address §

Greatbatch Medical, Clarence, NY 14031

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This article is dedicated to Prof. Ganapati D. Yadav as part of a Festschrift of Industrial & Engineering Chemistry Research. We gratefully appreciate assistance from Prof. Robert E. Baier (Department of Oral Diagnostic Sciences at the University at Buffalo) with contact angle measurements. We also appreciate thoughtful comments from the anonymous reviewers. We acknowledge support of this work from the New York State Pollution Prevention Institute and from the Indo−US Science and Technology Forum. Coating methodologies developed in this work were supported by the National Science Foundation under IIP 1311837.

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REFERENCES

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Nanostructured Magnesium Composite Coatings for Corrosion

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